1. Field of the Invention
The present invention relates to Wavelength Division Multiplexing (WDM) optical communications and, more particularly, to a method for maintaining the mode-locked state of a Fabry-Perot laser irrespective of the changes in peripheral temperature, and a WDM light source incorporating the same method.
2. Description of the Related Art
A WDM passive optical network (PON) generally provides high-speed, wideband communication services using unique wavelengths assigned to each subscriber. As such, the WDM-PON can secure communication confidentiality. Further, it can accommodate a communication-capacity extension requested by each subscriber and easily extend the number of subscribers by simply adding unique wavelengths to be assigned to the new subscribers. Despite these advantages, the WDM-PON has not yet been put to practical use as it imposes a heavy economic burden on subscribers due to the need to provide a central office (CO) and each subscriber terminal with a light source of a specific lasing or oscillation wavelength as well as an additional wavelength-stabilization circuit for stabilizing the wavelength of the light source.
Accordingly, the development of an economical WDM light source is essential to realize the WDM-PON. To this end, a distributed feedback (DFB) laser array, a multi-frequency laser (MFL), a spectrum-sliced light source, and a mode-locked Fabry-Perot (FP) laser with incoherent light have been proposed. However, the DFB laser array and the MFL require a complicated manufacturing process and are costly. In addition, a wavelength stabilization and a correct wavelength selection of the light source are essential to implement the wavelength division multiplexing. Recently, the spectrum-sliced light source has been developed to provide a number of wavelength-divided channels by spectrum-slicing a broad-bandwidth optical signal through an optical filter or a waveguide grating router (WGR). For example, a light emitting diode (LED), a superluminescent diode (SLD), a Fabry-Perot (FP) laser, a fiber amplifier light source, and an ultra-short pulse light source have been proposed, and these elements do not require the spectrum-sliced light source to employ a light source of a specific lasing wavelength as well as additional equipment for achieving wavelength stabilization.
Proposed as a spectrum-sliced light source, the LED and SLD are not expensive and also have a wide optical bandwidth. However, the LED and SLD are suitable for use as a light source for upstream signals having a lower modulation rate rather than downstream signals as they have a low modulation bandwidth and a low output power. The FP laser is a low-priced, high-output element, but has disadvantages in that it cannot provide a large number of wavelength-divided channels because of its low bandwidth, and its performance degradation due to the mode partition noise is serious when modulating and transmitting a spectrum-sliced signal at a high rate. The ultra-short pulse light source is coherent and also has a very wide light-source spectrum band, but its implementation is difficult as the lasing spectrum has low stability and its pulse width is only several picoseconds.
To address the deficiencies in the above light sources, a spectrum-sliced fiber amplifier light source has been proposed as a large number of high-power, wavelength-divided channels by spectrum-slicing ASE (Amplified Spontaneous Emission) light generated by an optical fiber amplifier. However, this light source must use an additional high-priced external modulator, such as a LiNbO3 modulator, for allowing the channels to transmit data different from each other.
Another proposed light source is known as a mode-locked Fabry-Perot (FP) laser with incoherent light which produces a mode-locked signal. In order to produce the mode-locked signal, after a wide-bandwidth optical signal is generated from an incoherent light source, such as an LED or a fiber amplifier light source, through a waveguide grating router (WGR) or an optical filter, it is spectrum-sliced and then injected into an FP laser having no isolator. When a spectrum-sliced signal of a predetermined power level or more is injected into the FP laser, the FP laser generates and outputs only the light of a wavelength coinciding with the wavelength of the injected signal. Such a mode-locked FP laser with incoherent light can perform data transmission more economically by directly modulating the FP laser according to a data signal.
However, a wide-bandwidth, high-power optical signal must be injected into the FP laser in order for the FP laser to output a mode-locked signal suitable for a high-speed, long-distance transmission. Further, in the absence of controlling external temperature, the Fabry-Perot laser mode is changed to another mode when the temperature varies. This mode change causes the Fabry-Perot laser to release from the locked state, escaping from a wavelength coinciding with the wavelength of the injected spectrum-sliced signal. Thus, the mode-locked Fabry-Perot laser cannot be adapted as a WDM light source. An external temperature controller (e.g., a TEC controller) is thus indispensable to adapt such a mode-locked Fabry-Perot laser when used as a WDM light source.
FIG. 1 shows the configuration of a conventional Fabry-Perot (FP) laser having a temperature controller. As shown, the conventional FP laser includes a TEC (Thermo-Electric Cooler) controller 1, a thermistor 2, an FP laser 3, and a TEC 4. The TEC controller 1 detects the temperature of the FP laser 3 through the thermistor 2 and controls the temperature of the FP laser 3 using the TEC 4.
The conventional FP laser, however, has an increased packaging cost because the thermistor and the TEC must be coupled to the FP laser, and the need to provide an additional TEC controller further increases the overall cost. These impose a high economic burden on subscribers, so that the WDM-PON has not yet been widely accepted.